|There's a lot of math involved in making something like this (which I'm not to good with), like calculating where to draw the player, where to draw zombies, zombie AI, xp and level system, combat system, weapon stats, etc.|
The ammo, health, xp and stuff can most likely be done using simple math /set /a ) (I'd recommend using larger values instead of smaller ones with decimals, as batch doesn't like those)
For making movement, check out this thread
If I were you, I'd sit down with a different language more suited to this kind of thing.
If you do want to stick with batch on this, I'd suggest you build up some proper knowledge about how commands and the command processor work, here are some excellent links:
Don't worry if plan A fails, there are 25 more letters in the alphabet ;)